Hot rolled steel sheets for extra deep drawing

ABSTRACT

A HOT ROLLED STEEL SHEET HAVING EXTRA DEEP DRAWING AND PRESS FORMABILITY PROPERTIES COMPARABLE TO COLD ROLLED STEEL SHEETS, WHEREIN THE STEEL IS VACUUM DEGASSES AND HAS A COMPOSITION CONSISTING ESSENTIALLY OF: 0.001-0.020% CARBON, LESS THAN 0.10% SILICON, 0.10-0.60% MANGANESE, LESS THAN 0.020% SULPHUR, 0.010-0.10% ALUMINUM, 0.020.08% TITANIUM, BALANCE IRON AND INCIDENTAL IMPURITIES AND WHEREIN TI/C&lt;4 AND TI/S&gt;2. THE STEEL SHEETS ARE HOT ROLLED WITH A FINISHING TEMPERATURE IN THE RANGE 800950*C. AND COLLED AT A TEMPERATURE IN THE RANGE 650*C.700*C.

HOT ROLLED STEEL SHEE'IS FOR EXTRA LEEF DRAWING F 1973 KAZUO MATSUDO T 2Sheets-Sheet Filed Nov. 8. 1971 jLFQ9). 13, 1973 KAZUO MATSUDQ TAL3,716,4Z0

HOT ROLLED STEEL SHEETS FOR EXTRA DEEP DRAWING Filed Nov. 8, 1971 2Sheets-Sheet 2 United States Patent Patened EIN,

percent YPE1, percent percent Mechanical property ABSTRACI OF THEDISCLOSURE A hot rolled steel sheet having extra deep drawing and pressformability propertes comparable to cold rolled steel sheets, whereinthe steel is vacuum degassed and has a composition consistingessentially of: 0.0010.020% carbon, less than 0.10% slicon, 0.100.60%manganese, less than 0.020% sulphur, 0.010-0.10% aluminum, 0.02- 200.08% titanium, balance iron and incidental impnrities and wherein Ti/C4 and T/S 2. The steel sheets are hot rolled with a finishingtemperature in the range 800- 950 C. and coiled at a temperature in therange 650 C.- 700 C.

Coiling tempera- Directnre, C. tion temperature, C.

This invention relates to steel sheets and in particular to hot rolledsteel sheets having a relatively 10W tensile strength and excellentpress forrnability.

Heretofore most extra deep drawing steels are produced by cold rollingprocesses, but snch processes are relatively expensive. It is,therefore, economically advantageous to provide hot rolled sheets fordrawing purposes provided press formability equal to that of cold rolledsheets can be achieved in sheets in the range of 1.63.4 mm. thick. Forexample, a 3.2 mm. thick sheet produced by cold rolling has anapproximate notch elongation of 20% but by conventional hot rolling fordrawing the notch elongation falls far short of that level. 4

It is, therefore, an object of the present invention to provide a hotrolled steel sheet having a notch elongation of at least 20% A number ofproposals have been made concerning provision of hot rolled steel sheetshaving excellent press formability. Notable among these are vacuurndegassed aluminnm klled steels and desulfurized low manganese aluminumkilled steels. Table 1 shows an example of each of the two types ofprior art hot rolled steels to- 0 gether with the heat treatments andmechanical properties 5 of such steels.

Applicants have found that similar or superior properties can beachieved in hot rolled steel sheets of 10-70 mm. thick provided suchsteels, are vacuum degassed and contain a carefully controlled additionof titanium.

Thus, by one aspect of the present nvention there is provided a steelconsisting essentially of 0.001-0.020% carbon, less than 0.10% silicon,0.100.60% manganese, less than 0.020% sulphur, 0.010-0.10% aluminum,0.02- 0.08% titanium, balance iron and incidental impurities, providedalways that Ti/C 4 and Ti/S 2.

By a preferred embodirnent the steel of the present invent is hot rolledat a finishing temperature between 800- 950 C. and coiled at atemperature between 650- 700 C.

Specific reasons for limiting the quantity of the components of the hotrolled steel sheets of this invention are explained in more detailhereafter.

Increases in carbon content cause increases in carbide precipitationwhich are in turn detrimental to the press formability of the steel. Inthe steel of the present invention, therefore, the carbon content isreduced as much as TABLE 1 Het rolling conditions Composition (percent)by weight nlelsstof Finshing s ee s, 81 Mn S N2 S01Al mm. r 0.332 0.0180.017 0.0029 0.043 3 2 0.01 0.18 0.010 0.012 0.0030 0.016 3.2

NOTEEIN represents notch elongation in a notch tenslle test whichturnishes a reliable standard for evaluatng press formability of thinsheets.

Items Degassed A1 killed steel 3 possible by vacuum degassing s as toachieve higher press formability. 0.001% carbon is the practical lowerlimit of decarbonization by vacuum degassing and the upper limit of0.020 represents the point above which the tensile strength of the steelrises sharply because of TiC precipitation.

Silicon is another element which increases the tensile strength of steelsheets and therefore this element is restrcted to amounts below 0.1%.

Manganese in an amount in excess of 0.1% is added in order to preventhot shortness caused by sulphur. The upper limit of manganese must,however, be controlled to EXAMPLE 1 Table 2 indicates the chemicalcomposition of steels covered by this invention and Table 3 indicatesthe manufacturing conditions and mechanical properties upon hot rollingof such steels.

TABLE 2 Percent b w h less than 0.6% because manganese works, hke carbonand C y silicon, t0 increase the tensile strength of the steel. Si TrSulphur is mantained at a level of less than 0.020% Mn 0.20 becauselarger amounts affect the cleanlrness of the steel P 0 011 and adverselyafiect the press formability beca se of the S 0012 formation of MnS.(10.040 Titanium is added to the steel because it C0nverts MnS 5,51 1from an A type to a C type impuri y. An A type incluson 20 T 0 03 P v m0r impur1ty 1S defined as one WhlCh 1s plastically deformed Ti/C 3 7 byworkjng, and a C type inclusion is defined as one which T/S 2.5

'IABLE 3 Hot: rolling conditions C.) Finishing Coling Skin- TestMechanical pmperty Thickness of plates tempertempcrpass, direc- YP, IS,E1, YPE1, E1N, AI, (mm.) ature ature percent tien lrg.lmm. kg./rnmflpercent percent percent kg.lmm 2 NOTE AI=Agng index.

is scattered at random throughout the matrix and which is notplastically defined by working. The C type inclusion is less detrimentalto press formability, and at the same time produces solid solution ofsulphur to some extent through the formation of TiN thereby serving todecrease MI1S-type sulfides extending in the direction of rolling. Thus,the Ti added to hot rolled steel sheets achieve excellent pressformability. It has been found that the Ti content must satisfy theratio Ti/S Z in order to achieve the above noted conditions. Inaddition, the ratio Ti/C 4 further serves to improve the pressformability of the hot rolled steel sheets provided the condition of T/S2 is also met. For the above reasons, the Ti content is fixed at0.020.08% hearing in mind its further balance with the carbon andsulphur contents.

Aluminum is added. in an amount of 0.010%0.10% in order to facilitatedeoxidation and eiect a better Ti yield. Amounts in excess of 0.10% aredetrimental to the press formability of the steel sheets, and an amountless than 0.010% lessens the effect of deoxidation.

In order to achieve the desired mechanical properties in the hot rolledsteel sheets for extra deep drawing, the heat treatrnent steps must becontrolled Within the temperature ranges previously discussed.

In order to produce steel sheets f0r extra deep drawing having excellentpress formability, it is desirable that finishing temperature of therolling is above the Ar transforrnaton temperature, because the pressformability rapdly deteriorates below the Ar temperature. However, theAr transformation temperature is in the vicinity of 850 C. in the caseof sheets of extremely low carbon content and t is difficult to maintaina finishing temperature as high as 850 C. in very thin sheets.I-Iowever, the elect of a finishng temperature below the Artransformation temperature is, at least in part, olset by carefulcontrol of the steel composition within the ranges previously discussed.Because the steels of the present invention range frorn 1.0 to 7.0 mm.thick, the range of finishing temperatures may be between 800 C. and 950C. In operations, it is desirable to maintain a finishing temperature ashigh as pos sible above the Ar transformation temperature. On the otherhand, the coilng temperature can be less than 650 C. and still proviclean acceptable press formabilty. Be-

As is clear from this example, steels manufactured in accordance withthis invention have low tensile strength and excellent pressformability.

In order to more clearly illustrate the differences between the steelsof the present invention and other hot rolled steel sheets for drawingof the same thickness, reference is made to the accompanying drawings in40 which:

FIG. 1 shows the relation between the notch elongation percentage andthe tensile strength respcctively of the products of the presentinvention and the prior art.

FIG. 2 is a microphotograph (100 magnifications) of the structure of asteel plate manufactured by the method of this invention and FIG. 3 is amicrophotograph (500 magnifications) of the structure of inclusions.

Differences in notch elongation between steels of this invention andother hot rolled steel sheets for drawing of the same thickness areshown in FIG. 1. It is obvious therefrorn that the press formability ofthe steel sheets of this invention is far superior to those of theconventional hot rolled steel sheets for drawing, an approach is that ofcold rolled steel sheets.

As shown in FIG. 2, the microphotograph of the ferrte structure of asteel sheet according to this invention shows an ideal formation. FIG. 3shows that Al O and MnS are closely attached to TN+TS. This gain showsthat excellent characteristics may be achieved by products of thepresent invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A vacuum degassed steel hot rolled nto sheets for extra deep drawing,said steel consisting essentially of: 0.001-0.020% carbon, less than0.10% silicon, 0.10 0.60% manganese, less than 0.020% sulphur, 0.010-0.10% aluminum, 0.02 0.08% titanium, balance iron and incidentalimpurties and whercin Ti/C 4 and Ti/S 2.

2. A steel sheet as claimed in claim 1 wherein said sheet is hot rolledwith a finishing temperature within the range of 800-950 C. and coiledat a temperature in the range of 650-700 C. whereby press formabilityproperties are enhanced.

5 3. A process for producing steel sheets for extra deep Reference5 Cteddrawing comprislng vacuum degassing a steel havng a UNITED STATESPATENTS composrtron c0nsrst1ng essentrally of 0.0010.020% carbon, lessthan 0.10% silicon, 0.100.60% manganese, less 3522110 7/1970 shlmlzu et14812 than 0.020% sulphur, 0.010-0.10% alumnum, 0.02 3560270 2/1971Rembold et 14812 5 3,67456 9/1971 Forand, Jr. 14812 0.08% titanium,balance iron and incidental mpurties and wherein T/C 4 and T/S 2, hotrolling said steel W AYL AND T ALL A P nto sheets 1.0-7.0 rnm. thickwith a rolling finishing S nmary Exammer temperature in the range of800950 C. and coling said U.S. C1. X.R.

steel sheet at a temperature in the range 650700 C. 10 148-12.1 36

